Generally in electrostatographic reproduction, the original to be copied is rendered in the form of a latent electrostatic image on a photosensitive member. This latent image is made visible by the application of electrically charged toner.
The toner thusly forming the image is transferred to a receiver, such as paper or transparent film, and fixed or fused to the receiver. The fusing of toner to receiver can be effected by applying heat, preferably at a temperature of about 90° C.–200° C.; pressure may be employed in conjunction with the heat.
A system or assembly for providing the requisite heat and pressure customarily includes a fuser member and a support, i.e., pressure member. The heat energy employed in the fusing process generally is transmitted to toner on the receiver by the fuser member. Specifically, the fuser member is heated. To transfer heat energy to toner situated on a surface of the receiver, the fuser member contacts the toner, and correspondingly also can contact the surface of the receiver itself. The support member contacts an opposing surface of the receiver. Accordingly, the receiver can be situated between the fuser and support members, so that these members can act together on the receiver to provide the requisite pressure in the fusing process.
During the fusing process, toner can offset from the receiver to the fuser member. Toner transferred to the fuser member in turn may be passed on to other members in the electrostatographic apparatus, or to subsequent receivers subjected to fusing.
Toner on the fusing member therefore can interfere with the operation of the electrostatographic apparatus and with the quality of the ultimate product of the electrostatographic process. This offset toner is accordingly regarded as contamination of the fuser member, and preventing or at least minimizing this contamination is a desirable objective.
U.S. Pat. No. 5,217,837 discloses a toner fusing system which utilizes internal heating with a fuser having a 30–65 micrometer thick fusing surface layer over a 1–3 millimeter thermally conductive HTV silicone elastomer layer. U.S. Pat. Nos. 5,017,432 and 5,332,641 disclose toner fusing systems which use internal heating and fusers with fluoroelastomer fusing surfaces.
Toner fusing systems using external heating are also known. U.S. Pat. Nos. 4,372,246, 4,905,050, 4,984,027, and 5,247,336 all disclose external heating for a toner fusing system. Of these, the latter three teach a configuration with the fuser roller situated between, and in contact with, two external heating rollers.
A factor in achieving sufficient fusing quality is providing sufficient heat transfer from the fusing surface layer of the fuser member to the receiver toner. This heat transfer could be improved by increasing the thermal conductivity of the fusing surface layer of the fuser member, and thermal conductivity can be increased by increasing the amount of heat conducting filler in the fuser member. In general, providing the fusing surface layer with at least a particular minimum amount of the filler is necessary to obtain sufficient thermal conductivity, heat transfer, and fusing quality.
However, heat conducting filler particles in the fusing surface layer provide high energy sites for removing toner from the receiver. Therefore, increasing the amount of heat conducting filler content in the fusing surface layer, by providing more reactive sites for the toner, can increase toner offset and therefore also increase contamination of the fuser member.
It would be desirable to have a fuser roller that improves fuser roller wear resistance while maintaining desirable fusing. It would further be desirable that the fuser roller use external heating and prevent offset from occurring. It would still be further desirable to maintain good paper handling.